PROJECT SUMMARY Many exciting therapeutic interventions for Duchenne muscular dystrophy are emerging, but the challenges of quantitating functional dystrophin are complicating drug approvals and delaying the pace of clinical translation. To overcome this obstacle, we are developing a multiplexed imaging-mass spectrometry (I-MS) method to simultaneously quantitate and localize dystrophin and other members of the DGC in single muscle sections. Our preliminary data showing successful quantitation and localization of dystrophin and myosin in mouse muscle sections supports the feasibility of our proposed approach. This approach improves on the quantitation of Western blots and simultaneously provides the localization of dystrophin and other DGC proteins that was previously done by separate immunohistochemistry. Specific Aim 1: Development and validation of rare earth element labeled antibodies for simplex imaging-mass spectrometry of dystrophin and other DGC components. 32 primary antibodies, DNA intercalators or lectins will be labeled with specific REEs. Each REE-labeled antibody will be validated individually by comparing Western blot, immunofluorescence and I-MS using wild-type, heterozygous mdx, homozygous mdx mouse and normal human muscle samples. This aim will: 1) develop a robust pipeline for antibody labeling, quality control and validation for use in I-MS, 2) benchmark this new approach to conventional approaches, and 3) provide the solid foundation required for multiplex I-MS in Aim 2. Specific Aim 2: Multiplex imaging-mass spectrometry of dystrophin and other DGC components. We will assemble a panel of the validated antibodies from Aim 1 and use them to simultaneously measure dystrophin and other DGC components in single muscle tissue sections. We have selected tissues of varying dystrophin and DGC component levels including: 1) transgenic mouse models engineered for increasing levels of human sarcospan, 2) patient carriers of a mutant dystrophin gene and 3) patients with Becker's muscular dystrophy. This aim will: 1) optimize and validate the multiplexing of reagents from Aim 1, 2) test the sensitivity of this approach on clinically relevant samples and 3) demonstrate clinical translatability. Our novel application of imaging-mass spectrometry will facilitate the absolute quantitation and sarcolemmal localization of functional dystrophin and up to 32 DGC members in very small muscle samples. Clinical research in DMD will benefit from the ability to accurately quantitate and image functional dystrophin and the DGC, which will make the best use of valuable patient resources, provide improved information on drug efficacy and safety, and speed the pace of clinical translation.